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Stock-based Compensation
 12 Months Ended
Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]  
Stock-based Compensation Stock-based Compensation
2015 Stock Plan
The 2015 Stock Plan, as amended (the “2015 Stock Plan”), is our shareholder-approved incentive plan through which equity based grants are awarded. The 2015 Stock Plan provides for the grant of incentive stock options, non-statutory stock options, restricted stock, restricted stock units and other stock-based compensation awards to employees, officers, directors, and consultants of the Company. The administration of the 2015 Stock Plan is under the general supervision of the Compensation Committee of the Board of Directors. The terms of stock options awarded under the 2015 Stock Plan, in general, are determined by the Compensation Committee, provided the exercise price per share generally shall not be set at less than the fair market value of a share of the common stock on the date of grant and the term shall not be greater than ten years from the date the option is granted. As of December 31, 2023, 711,054 shares were available for future issuance under the 2015 Stock Plan.
2023 Inducement Plan
In September 2023, the Company adopted the 2023 Inducement Plan (the “Inducement Plan”), pursuant to which the Company may from time to time make equity grants to new employees as a material inducement to their employment. The Inducement Plan was adopted without stockholder approval, pursuant to Nasdaq Listing Rule 5635(c)(4), and is administered by the Compensation Committee of the Board. The Inducement Plan provides for the granting of non-statutory stock options, restricted stock, restricted stock units, performance stock units and other stock-based compensation awards to new employees, but does not allow for the granting of incentive stock options. The terms of the stock options under the Inducement Plan, in general, are determined by the Compensation Committee, provided the exercise price per share generally shall not be set at less than the fair market value of a share of the common stock on the date of grant and the term shall not be greater than ten years from the date the option or award is granted. A total of 500,000 shares of the Company’s common stock were reserved for issuance under the Inducement Plan. As of December 31, 2023, 193,392 shares were available for future issuance under the 2023 Inducement Plan.
Stock Options
The following table summarizes stock option activity during the year ended December 31, 2023:
Shares
Weighted
average exercise
price		Weighted
average
remaining
contractual life
(years)		Aggregate
intrinsic value
(in thousands)
Outstanding at January 1, 20232,857,054$81.78 
Options granted31,111190.91 
Options exercised(460,385)88.83 
Options cancelled(72,001)144.24 
Outstanding at December 31, 20232,355,779$79.94 5.51$359,861 
Exercisable at December 31, 20231,774,976$74.49 4.71$281,400 
The total cash received by the Company as a result of stock option exercises was $34.0 million, $9.0 million and $1.0 million for the years ended December 31, 2023, 2022, and 2021. The total intrinsic value of options exercised was $70.4 million $47.3 million and $0.1 million for the years ended December 31, 2023, 2022, and 2021. The weighted-average grant date fair values, based on the Black-Scholes option model, of options granted during the year ended December 31, 2023, 2022 and 2021 was $149.15, $54.68, and $73.29, respectively.
Restricted Stock Units
The Company’s 2015 Stock Plan provides for awards of restricted stock units (“RSUs”) to employees, officers, directors and consultants to the Company. The Company’s Inducement Plan provides for awards of RSUs to new employees. RSUs vest over a period of months or years, or upon the occurrence of certain performance criteria or the attainment of stated goals or events, and are subject to forfeiture if employment or service terminates before vesting.
The following table summarizes RSU activity, excluding performance-based RSUs, during the year ended December 31, 2023:
Shares
Weighted
average grant date fair value
Unvested at January 1, 2023—$— 
RSUs granted
398,600243.81 
RSUs vested
(356)299.98 
RSUs forfeited
(22,127)283.05 
Unvested at December 31, 2023376,117$241.45 
Performance-Based Restricted Stock Units
The Company has granted various performance-based restricted stock units (“PSUs”) to certain senior leadership. Depending on the terms of the PSUs and the outcome of the pre-established performance criteria, which may include a market and/or performance condition, a recipient may ultimately earn the target number of PSUs granted or a specified multiple thereof at the end of the vesting period.
As of December 31, 2023, the Company granted 50,000 PSUs. The maximum number of PSUs eligible to be earned of 150,000 are outstanding under the Inducement Plan as of December 31, 2023, with a weighted average grant date fair value of $146.37 per unit.
Outstanding Awards
As of December 31, 2023, the Company had restricted stock units, performance stock units, and options outstanding pursuant to which an aggregate of 2,881,896 shares of its common stock may be issued pursuant to the terms of all awards granted under the 2015 Stock Plan and Inducement Plan.
Stock-Based Compensation Expense
Stock-based compensation expense during the years ended December 31, 2023, 2022 and 2021 was as follows (in thousands):
Year Ended December 31,
202320222021
Stock-based compensation expense by type of award:
Stock options$30,613 $31,625 $26,873 
Restricted stock units
14,974— — 
Performance-based restricted stock units
4,148— — 
Total stock-based compensation expense$49,735 $31,625 $26,873 
Effect of stock-based compensation expense by line item:
Research and development$20,864 $13,876 $10,698 
General and administrative28,871 17,749 16,175 
Total stock-based compensation expense included in net loss$49,735 $31,625 $26,873 
Unrecognized stock-based compensation expense as of December 31, 2023 was $118.2 million with a weighted average remaining period of 2.66 years.